This invention relates to improved internal combustion engines, and especially to two-stroke engines having improved lubrication, scavenging, charging and exhaust port timing to reduce polluting emissions and improve engine performance and fuel efficiency.
The present invention recognizes the global need for reduced hydrocarbon emissions from small power-producing engines, especially as relates to the rapidly growing demand for agricultural and light industrial power in developing economies. In these economies, the low weight and low cost of two-stroke engines will be difficult to ignore, and it may be expected that two-stroke engines will be widely used. Two-stroke engines have inherently high levels of unburned hydrocarbon emission due to their operating principle, in which burned exhaust gases are expelled from the engine""s cylinder at the same time that a fresh fuel/air charge is brought in, leading inevitably to mixing between the two and inadvertent expulsion of unburned charge with the exhaust gases.
Furthermore, two-stroke engines pass their fuel/air charge through the crankcase in order to allow a slight pressurization, caused by the descent of the piston, to assist the flow of charge into the cylinder. As it passes through the crankcase, the charge entrains lubricating oil droplets, which are splashed on the crankshaft main and rod end bearings and sprayed on the cylinder walls and wrist pin. (Alternately, oil is mixed with the fresh charge before entering the crankcase, in which case the charge is used as an agent for transporting oil to the surfaces requiring lubrication.) Lubricating oil entrained in the charge is inducted into the cylinder, where it either flows through into the exhaust, creating more unburned hydrocarbon emission, or stays in the cylinder and is burned, creating a more noxious set of pollutants than would stem from the combustion of the engine fuel itself.
The pollution disadvantages of conventional two-stroke, spark-ignited engines (overlap of intake and exhaust flows and crankcase charge compression) lead to its advantages in day-to-day applications. Since the exhaust and intake strokes are not separate, for a given requirement for engine power and speed, at a gas constant compression ratio, a two-stroke engine requires only half the displacement of a four-stroke engine. The weight of the two-stroke engine would also be little more than half of the weight of a power-equivalent four-stroke engine and cost much less to produce. These advantages will prove very difficult to ignore in a developing economy, and thus, if two-stroke engines retain their conventional form, there is a great potential for globally significant increases in engine-related air pollution.
The present invention retains the engine size advantage of the two-stroke engine, the cost advantage of the carbureted two-stroke engine and reduces its unburned hydrocarbon emissions and lubricating oil combustion characteristics to levels comparable with the most advanced direct injected, two-stroke, dry-sump engines. This is accomplished with a relatively minor increase in cost for the inclusion of new parts and new machined or cast features on conventional parts. These parts and features allow the present invention, an improved two-stroke, spark-ignited engine, to operate with very little unburned fuel emission and with very little lubricating oil combustion.
Nearly complete reduction in unburned fuel emission is achieved in the invention by separating the air flow from the crankcase to the cylinder into two separate tubes. One tube contains air only and scavenges the burned gas out of the cylinder through the exhaust port. The top of the port for this scavenging tube is located relatively high on the cylinder wall and is uncovered sooner on the piston down-stroke. The other tube contains air and fuel and charges the cylinder. The top of the port for the charging tube is located relatively lower on the cylinder wall and is uncovered later on the piston down-stroke than the scavenging tube port. This timing of the ports will allow the air-only scavenging flow time to purge the cylinder of burned gas before the air/fuel charge flow is initiated. Fuel will not be mixed with air on inlet to the crankcase, as is the case in conventional two-stroke spark-ignited engines, but rather is mixed on the passage from the crankcase to the cylinder through the charging tube. Fuel is mixed with air only on its passage through this charging tube and not on its passage through the scavenging tube.
Not all of the scavenging air is exhausted from the cylinder, and the remainder is mixed with the air/fuel charge. Therefore, in order to maintain an appropriate overall air/fuel ratio in the cylinder, the charging air/fuel mixture must be rich in fuel. This rich charge can be directed, with appropriate port and tube design, towards the spark plug. Combustion is then initiated, at the plug, in a rich mixture (mostly rich charge and a little scavenge air) and burns out, away from the plug, into a lean mixture (mostly scavenge air and a little rich charge). This is precisely the principle behind stratified charge ignition, a widely recognized enhancement to combustion efficiency, pollution reduction and engine cycle efficiency. This sort of rich-lean combustion cannot be achieved in a conventional two-stroke, spark-ignited engine. It is achievable in the present improved two-stroke, spark-ignited engine because of the novel features of the invention. This advantageous form of combustion is also achievable using advanced, direct-injected, two-stroke engine technology; however, direct injection of fuel into the cylinder is costly, and such a system would be difficult to acquire and maintain in a developing economy. The present invention allows the advantages of stratified combustion while using only easily achieved, relatively low-cost technologies.
A further achievement of the invention""s separated charging and scavenging flows is that the engine may be controlled by throttling only the charging flow. As a result, the present improved two-stroke spark-ignited engine will have higher partial-load efficiency than conventional two-stroke spark-ignited engines. Conventional spark-ignited engine control is achieved by throttling the intake flow, which reduces the amount of fuel entering the engine and also reduces the amount of air intake. These reductions are achieved by partially blocking (throttling) the intake flow, leading to large pressure drops in the intake flow and reduced engine efficiency due to the piston-cylinder pumping power needed to overcome this pressure drop. In the present improved two-stroke, spark-ignited engine, intake flow is divided into separate charging and scavenging flows. At partial-load only the charging flow needs to be throttled, leaving the scavenging flow without any pressure drop, and reducing the total amount of pumping power needed at partial-load, thus, increasing the engine""s efficiency. This advantage in engine efficiency is also achievable using advanced, direct-injected, two-stroke engine technology; however, direct injection is costly and would be difficult to acquire and maintain in a developing economy. The present invention allows high efficiencies while using only easily achieved, relatively low-cost technologies.
Nearly complete reduction in lubricating oil combustion is achieved in the invention by using a novel system for dry-sump lubrication, in which oil is circulated by piston pumping action (assisted by a crankshaft-mounted pump if necessary) from a reservoir that is segregated from the crankcase by seals. The oil passes through and lubricates bearings in the crankcase via a system of sealed passages or conduits. Oil is pumped through these conduits by the novel arrangement of an oil sleeve mounted between the cylinder and the crankcase, a fixed oil seal positioned between the oil sleeve and the cylinder and a moving oil seal mounted on the piston. An annular oil space is defined between the piston and the oil sleeve. A small, controlled amount of oil is allowed to escape past the fixed seal, up into the cylinder, in order to lubricate the compression rings and then be consumed, as is normal practice in engine design. The remainder of the oil is circulated by the pumping action of the moving seal against the fixed seal, which forces lubricating oil from the piston-cylinder annulus into the engine""s internal passages, lubricating the wrist pin, the cylinder wall, the rod end bearings and the main bearing. (Alternately, the fixed seal may be a sealing ring as well, hence forcing all of the oil from the annular oil space into lubrication passages in the wrist pin; a small flow of oil to lubricate the compression rings may be drawn from the wrist pin through internal passages in the piston.) Oil returns from the main bearing to the reservoir to complete its cycle. The crankcase remains dry, separated by shaft seals from the oil reservoir.
In a conventional two-stroke engine, oil is either broadcast as a spray throughout the crankcase or inducted as a mist with the charge air. In both cases, the lubrication points are serviced by filling the entire crankcase with oil droplets. Many of these are inevitably inducted into the cylinder. In the lubrication system of the invention, oil is only distributed to surfaces where it is needed for lubrication, and oil droplets do not enter the charge air stream. Therefore, lubricating oil consumption is limited to small amounts spread on the cylinder walls and seeping through the piston ring gaps, as would be typical of a four-stroke engine. The lubrication system of the invention greatly reduces the excessive oil combustion and unburned emission of conventional two-stroke engines (especially at idle speeds), which has reduced two-stroke acceptance on environmental grounds. The invention""s lubrication system makes the task of premixing oil and fuel unnecessary and avoids the loss of lubricating potential attendant to dilution with fuel. Employment of the invention should lead to a reduction in lubricating oil consumption, thereby lowering the operating cost of such engines. The lubricating system also reduces spark plug fouling and combustion chamber carbon deposits, because very little lubricating oil is burned in the cylinder. The reduction in oil consumption in the cylinder inherent in dry-sump lubrication might make it feasible to equip the present invention with a catalytic converter. Catalytic converters are not used on conventional two-stroke engines because they become fouled with oil emitted from the cylinder.
A variable exhaust port timing mechanism is incorporated into the engine according to the invention to further reduce any potential for emission of unburned hydrocarbons. As the piston descends on the power stroke, the exhaust port is the first port to be uncovered to initiate the blow-down process of releasing the cylinder pressure and initiating the exhaust flow. Therefore, the exhaust port upper lip is positioned highest in the cylinder of all the port lips, and for this reason, the exhaust port is also the last port to close. Even with good intake stratification and flow field tailoring, some fuel may be expected to flow out the exhaust port, as the piston rises after the charging and scavenging ports are sealed off. To inhibit this outward flow of fuel, a movable valve is incorporated into the exhaust port. This valve is lifted as the piston descends on the power stroke, thereby raising the position of the exhaust port upper lip and allowing early exhaust port opening. The valve is dropped before the piston""s subsequent ascent on the compression stroke, lowering the effective position of the exhaust port upper lip and allowing early (as opposed to late) exhaust port closing. Early exhaust port closing is a particularly suitable feature in the present invention. Since the scavenging flow precedes the charging flow into the cylinder (unlike in conventional two-stroke engines, in which these flows are coincident), fuel density in the cylinder is lower earlier in the piston""s cycle and higher later in its cycle. Therefore, if the exhaust port in the engine according to the invention is wider than in a conventional two-stroke engine but closes earlier, then it will only be open during periods of low fuel density, reducing the possibility of unburned fuel flowing out the exhaust port, while allowing the same total amount of exhaust flow.
In its preferred embodiment, the invention concerns an internal combustion engine having a piston reciprocable within a bore of a cylinder. The piston is pivotally connected to a crankshaft by a piston rod having a wrist pin at one end engaging the piston and a crank bearing at the opposite end engaging a throw of the crankshaft. The crankshaft is rotatably mounted on a main bearing within a crankcase positioned beneath the cylinder bore. An oil reservoir is mounted on the engine.
An oil sleeve is positioned between the cylinder and the crankcase, the oil sleeve having a bore therethrough coaxially aligned with the cylinder bore and sized to receive the piston. The piston is reciprocable within the oil sleeve bore. An annular oil space is defined between the piston and the oil sleeve.
A first seal is positioned between the cylinder and the oil sleeve, the first seal circumferentially surrounding the piston and permitting a predetermined amount of oil to flow from the annular oil space to the cylinder bore for lubricating the piston within the cylinder. A second seal is mounted on and circumferentially around the piston between the wrist pin and the crankshaft, the second seal having an outer circumference engaging the oil sleeve to substantially prevent oil within the annular oil space from flowing into the crankcase. A first conduit extends between the oil reservoir and a first port within the oil sleeve in fluid communication with the annular oil space. The first port is positioned so as to always be between the first and second seals regardless of the position of the piston within the cylinder bore and the oil sleeve bore. A check valve is mounted within the first conduit to prevent oil back flow from the annular oil space to the reservoir.
A second conduit extends along the wrist pin from a port in fluid communication with the annular oil space to the piston rod and along the piston rod to the crank bearing and from the crank bearing to the main bearing and then to the oil reservoir. Upon motion of the piston moving the second seal away from the first seal, lubricating oil is drawn from the oil reservoir through the first conduit and into the annular oil space to lubricate the piston. Upon further motion of the piston moving the second seal toward the first seal, lubricating oil within the annular oil space is forced through the second conduit back to the oil reservoir in a closed loop circulation, thereby lubricating the wrist pin, the crank bearing and the main bearing.
It is an object of the invention to provide an improved internal combustion engine with reduced hydrocarbon emissions.
It is a further object of the invention to provide a two-stroke or a four-stroke engine which will operate in any position, attitude or orientation.
It is another object of the invention to provide an improved engine having increased fuel and oil economy.
It is another object of the invention to provide a two- or four-stroke engine having a dry-sump lubrication system.
It is another object of the invention to provide a two-stroke engine having a scavenging air flow separate from a charging flow.
It is another object of the invention to provide a two-stroke engine with a stratified charge, having a relatively rich mixture near the spark plug.
It is another object of the invention to provide a two-stroke engine wherein the charging flow is throttled.
It is another object of the invention to provide a method of operating a two-stroke engine wherein the scavenging flow occurs before the charging flow.
It is another object of the invention to provide an engine wherein the piston pumps oil from a reservoir to lubricate the engine.
It is another object of the invention to provide an engine having variable exhaust valve timing.
These as well as other objects and advantages of the invention will become apparent from consideration of the following drawings and detailed description of the preferred embodiments of the invention.